Solid state image sensors are now used extensively in many types of image capture applications. The two primary image sensor technologies utilized are Charge Coupled Devices CCD and CMOS x-y addressable devices. Currently, there exists many different specific embodiments of both technologies, including Active Pixel Sensors (APS) and Passive Pixel Sensors (PPS) for CMOS x-y addressable devices. All are basically comprised of a set or array of photodetectors that convert incident light into an electrical signal that can be readout and used to construct an image correlated to the incident light pattern. The exposure or integration time for the array of photodetectors can be controlled by well known mechanisms. The signal represents the amount of light incident upon a pixel photosite. The dynamic range (DR) of an imaging sensing device is defined as the ratio of the effective maximum detectable signal level, typically referred to as the saturation signal, (Vsat), with respect to the rms. noise level of the sensor, (σnoise). This is shown in Equation 1.Dynamic Range=Vsat/σnoise  Equation 1:
Image sensor devices such as charge coupled devices (CCD) that integrate charge created by incident photons have dynamic range limited by the amount of charge that can be collected and held in a given photosite, (Vsat). For example, for any given CCD, the amount of charge that can be collected and detected in a pixel is proportional to the pixel area. Thus for a commercial device used in a megapixel digital still camera (DSC), the number of electrons representing Vsat is on the order of 13,000 to 20,000 electrons. If the incident light is very bright and creates more electrons that can be held in the pixel or photodetector, these excess electrons are extracted by the anti-blooming mechanism in the pixel and do not contribute to an increased saturation signal. Hence, the maximum detectable signal level is limited to the amount of charge that can be held in the photodetector or pixel. The DR is also limited by the sensor noise level, σnoise. Due to the limitations on Vsat, much work has been done in CCD's to decrease σnoise to very low levels. Typically, commercial megapixel DSC devices have a DR of 1000:1 or less.
The same limitations on DR also exist for APS and PPS devices. The Vsat is limited by the amount of charge that can be held and isolated in the photodetector. Excess charge is lost. This can become even more problematic with APS and PPS compared to CCD due to the active and passive components within the pixel, limiting the area available for the photodetector, and due to the low voltage supply and clocks used in CMOS devices. In addition, since APS devices have been used to provide image sensor systems on a chip, the digital and analog circuits used on APS devices such as timing and control and analog to digital conversion, that are not present on CCD's, provide a much higher noise floor on APS devices compared to CCD. This is due to higher temporal noise as well as possibly quantization noise from the on-chip analog to digital converter.
In commonly assigned U.S. Pat. No. 6,069,377, issued May 30, 2000, entitled IMAGE SENSOR INCORPORATING SATURATION TIME MEASUREMENT TO INCREASE DYNAMIC RANGE, by Prentice et al., Prentice discloses the prior art approaches to extending dynamic range of APS devices, and discloses a new invention to extend dynamic range. This method has the disadvantage of requiring more than four transistors per pixel and limits the size of the pixel that can be made. In U.S. Pat. No. 6,307,195, issued Oct. 23, 2001, entitled VARIABLE COLLECTION OF BLOOMING CHARGE TO EXTEND DYNAMIC RANGE, and U.S. Pat. No. 6,486,504, issued Nov. 26, 2002, entitled CMOS IMAGE SENSOR WITH EXTENDED DYNAMIC RANGE, both by Guidash, Guidash discloses extending dynamic range by collection of the charge that blooms from the photodetector, and by co-integration of the photodetector and floating diffusion within a single pixel. These approaches have the potential disadvantage of spatial variation of the photodetector saturation level contributing to fixed pattern noise in the sensor, and does not increase the sensitivity of the sensor.
Prior art APS devices also suffer from poor sensitivity to light due to the limited fill factor induced by integration of active components in the pixel, and by loss of transmission of incident light through the color filter layer placed above the pixel.
From the foregoing discussion it should be apparent that there remains a need within the prior art for a device that retains extended dynamic range while retaining low fixed pattern noise, small pixel, and high sensitivity.